The present invention relates to a laser processing apparatus and method, and more specifically, to a laser processing apparatus and method for enhancing processing efficiency of an object by means of a polygon mirror.
In producing objective materials from wafers, metals, plastics, and so on, it is general to operate processing procedures such as cutting and grooving works. For instance, after completing a semiconductor chip fabrication process, a process for cutting a wafer to separate plural chips, which are formed on the wafer, into individual chips is subsequent thereto. The wafer cutout operation is very important over the whole process of semiconductor chips because it heavily affect the productivity and product quality in the subsequent process. The wafer cutout operation is usually carried out with a mechanical cutout method or a method employing a laser beam. Especially, a processing apparatus using a laser beam is being highly studied because it has more advantageous than a mechanical apparatus.
A method of cutting a wafer by a laser beam uses a mechanism of burning irradiated areas out of the wafer due to inducing thermal and chemical effects by focusing the laser beam on a surface of the wafer in the range of high ultraviolet rays 250˜360 nm. In other words, when a laser beam condensed is irradiated on a wafer, the irradiated area is heated up instantly and then evaporated, melting as well, with increasing a vaporizing pressure according to the evaporation of the wafer material, resulting in an explosive burning-out of the irradiated area. From a successive sequence of the burning-out operations, a wafer can be divided into multiple chips and a linear or curved severing process is available therein along a moving passage.
The most advanced one among laser processing apparatuses is carried out by cutting a wafer with guidance by water ejected from a high-pressure water jet nozzle, irradiating a laser beam simultaneously. However, such an apparatus has a disadvantage of changing the nozzle in a period because the water jet nozzle is easily worn out mechanically due to the high pressure, which results in degrading the productivity and raising the costs as well as causing a nuisance in operating the process.
As one of techniques to overcome the defect in laser processing apparatuses at present, the method of employing a polygon mirror has been proposed in Korean Application No. 2004-0022270 filed on Mar. 31, 2004 by the present applicant. The polygon mirror rotates with plural reflection planes that have the same length each other, as illustrated in FIG. 1.
FIG. 1 is a diagram illustrating a laser processing method with a polygon mirror.
As shown in FIG. 1, the laser processing apparatus with the polygon mirror includes a polygon mirror 10 rotating on an axis 11 with plural reflection planes 12, and a lens 20 condensing a laser beam reflected on the reflection plane 12 of the polygon mirror 10. Here, while the lens 20 may be implemented with a telecentric f-theta lens installed in parallel with a stage 30 on which an object 40 (e.g., a wafer) to be cut out is settled. And it may be implemented with plural lenses as well, but FIG. 1 shows a sheet of lens for convenience' sake.
Along the rotation of the polygon mirror 10, the laser beam is reflected on the front of the reflection plane 12 as shown in FIG. 1A and then incident on the left end of the lens 20. The reflected laser beam is condensed on the lens 20 and then perpendicularly irradiated onto a designated position S1 of the object 40.
Continuously, as illustrated in FIG. 1B, according to further rotation of the polygon mirror, the laser beam is reflected on the center of the reflection plane 12 and then incident on the center of the lens 20. The incident laser beam is condensed on the lens 20 and then perpendicularly irradiated onto a designated position S2 of the wafer 40.
Next, as shown in FIG. 1C, the polygon mirror more rotates to make the laser beam reflected on the rear of the reflection plane 12. The reflected laser beam is incident on the right end of the lens 20 and then condensed on the lens 20 to be perpendicularly irradiated onto a designated position S3 of the wafer 40.
As such, the laser beam is irradiated onto the positions from S1 to S3 on the wafer 40 along the rotation of the polygon mirror. The range from S1 to S3 is referred to as a scanning length SL processed by the reflection plane 12 of the polygon mirror 10. In addition, an angle set by the laser beams each reflected on the front and rear of the reflection plane 12 is referred to as a scanning angle θ.
In the laser processing apparatus with the polygon mirror stated above, the laser beam is irradiated on the object with energy about 10 W. But, it may occur that excessive irradiation of the laser beam onto the same position of the object damages the object to degrade the reliability of the process.
Moreover, as it is conventional to use a laser beam with a predetermined diameter other than a pointing beam, the laser beam incident on the corners of the reflection plane may be partially dissipated (or lost in energy). Such energy loss due to dissipation of the laser beam is described with reference to FIG. 2 as follows.
FIG. 2 is a diagram illustrating the phenomenon of the energy loss at the corners of reflection planes of the polygon mirror.
As shown in FIG. 2, while applying the laser beam to the polygon mirror 10 that is rotating, if the laser beam with a fixed diameter is incident on the corner of the reflection plane 12, a part of the laser beam, an energy-reduced laser beam A, is reflected on the reflection plane 12 and applied to the lens 20. However, the other part of the laser beam, a lost laser beam B, is scattered on the reflection plane 12′ and thereby is not applied to the lens 20.
Therefore, the lens 20 condenses the energy-reduced laser beam A only to irradiate it to the object 40 mounted on the stage 30. As a result, there are differences in the processing efficiency between the area of the object exposed to the laser beam reflected on the corner of the reflection plane 12 and the other areas. It causes an irregular processing profile to result in the degradation of the processing reliability. Moreover, as the energy of the laser beam is dissipated at the corners, the resources are wasted.